5 Most Common PoE Questions

By: Brian Rhodes, Published on Feb 02, 2017

In our IPVM IP Networking course, Power Over Ethernet (PoE) is a core component that students show significant interest in.

The top ones are:

  1. Can I accidentally double PoE wattage by using midspans & switches together?
  2. Does each port produce max rated wattage?
  3. Can a cable plugged into a port, but not a camera, electrocute me or be a safety hazard?
  4. How far can PoE travel on a cable?
  5. Will cameras using power supplies be damaged by also plugging them into PoE ports?

In the sections below, we answer each question.

** ******* ** ********** ******, ***** **** ******** (PoE) ** * **** component **** ******** **** significant ******** **.

*** *** **** ***:

  1. *** * ************ ****** PoE ******* ** ***** midspans & ******** ********?
  2. **** **** **** ******* max ***** *******?
  3. *** * ***** ******* into * ****, *** not * ******, *********** me ** ** * safety ******?
  4. *** *** *** *** travel ** * *****?
  5. **** ******* ***** ***** supplies ** ******* ** also ******** **** **** PoE *****?

** *** ******** *****, we ****** **** ********.

[***************]

Question: "*** * ************ ****** *** ******* ** ***** ******** & ******** ********?"

******: **. *** ******* of ******* ***** *** involves * *********** ******* where * ****** ********** and ******** *** ***** from * ****** **** a ****** ** ******* injector. ******* ***** ****** devices ** *** ******* power **** ********* *******, they ** *** ********** receive *** *** *****. In **** ***, *** 'only' *** ******* ** the ***** ** **** by *** ****** ******* to *** *** ******* camera ** ******** ******.

** ****** **** ******** in ****** ******* **** ****** Tested****** *** ******** *** mechanics ** **** ******.

Question: "**** **** **** ******* *** ***** *******?

******: ** ** *** guaranteed. ***** * **** may ** ***** ** deliver *** *******, (**: 15.4W *** ***.*** ** 60W *** ***.***) *** ability ** *** *** to ******* ** ******* on *** ***** ****** of *** ****** *** maximum ********* ***** *********. In **** *****, ****** outpaces ******, ******* *********** issues ** ******** ********** for *** *******.

*** *******, **** ******** grade *** ****** (****** **-*******) *** *** ********* output *****:

*** *** *** ***** available ** *** ****** is ***. **** * PoE *****, **** *** power ** ******* ******* each, **: *** / 4 ***** = **.*** per ****. *******, *** max *** ***** ********* per **** ** ***** at **.** *** ***.*** to **.* * * 4 = **.**. *** difference ******* ******* **** specifications *** *** ****** power ********* ** *** switch ** * **** 8.6W. **** ***** ** we *** * ******* that ******** *** ****, the ***** ****** ***** be *********.

Question: "*** * ***** ******* **** * ****, *** *** * ******, *********** ** ** ** * ****** ******?"

******: **. *** ** the ******* *********** *******, PoE ***** ** *** actively ****** ****** * connected ****** ******** **. This ***** **** * cable ********* ** * PSE ** *** '***********' at *** ***** ******* in ** * *** device *** **** *** present * ****** ****** because ** ********** *******.

Question: "*** *** *** *** ****** ** * *****?"

******: *** ******* **** described ** *** ******** IEE802.3 ********. ** ******, power **** ****** ** far ** *** ******* length ***** *** ** networked. ** *******, **** maximum ****** ** **** farther, *** ***** ****** **** ******** Ethernet ****, ***** **** *** voltages **** ******** * full ****' **** **** the ******, ****** *** point *** **** ***** travel ** *** **** connected *****.

***** *** ** ***** for *** *** ***** distance, *** ******** ******* than **** **** *** meet ******** *********, *** additional ******* **** *** be ********* *** *** void ******* ********** ** used.

Question: "**** ******* ***** ***** ******** ** ******* ** **** ******** **** **** *** *****?"

******: *** ******, *** beware. ** **** *****, cameras ** ***** *** devices **** *** ******* power **** **** ********* from * *** ** the ****** ** ******* receiving ***** **** * low-voltage ***** ******. *******, especially **** ***** *** devices, ************ *** **** against ***** **** ** the **** ** ******** the ******.

** *******, **** ** not ** ***** **** newer *******, *** *** disclaimers ******* **** ********* should ** ******** ******.

PoE *** ************ ************* ** ******** **** to ***** ** *******, eliminating * ******** ***** and ****** *** *****. In **** *****, * PoE ****** ******** **** switching *** ***** ****** into * ****** ****. However, *** ***** ***-*** switches ** ***** *** demand ********* ******, * separate **** ****** * midspan ** ***** ****. More:***: ****** **. ************* ***** *** ** Video ************.

Comments (18)

"Can a cable plugged into a port, but not a camera, electrocute me or be a safety hazard?" Answer: NO...

That's not necessarily true. There are plenty of switches that have passive POE in which case there is no negotiation happening first. One popular example are the Ubiquiti Tough Switches, if the POE is activated on a port, voltage is always present and you can feel it, ask me how I know ;)

Products that use passive PoE are uncommon because of this.

I've always wondered what the reasons were for using passive PoE. Are there any advantages to it? I seem to see it more wireless products like ubiquiti.

I think it must be because of some legacy interpretations of pre- or early IEEE PoE 802.3af. There was ambiguity in the negotiation process that some manufacturers implemented differently.

Even when wattage is produced on the line, it is not generally full strength. There is still a 'negotiation' process that happens when the device properly identifies which Class it needs.

For 'proprietary' (ie: Ubiquiti only) PoE, all devices are designed to use the same PSE design . So the Ubiquiti version you mention may not adhere to IEEE 802.3 standards regardless. Non-standard PoE is sometimes used.

As time passes, PoE for video (especially with 60W or 100W versions come online), passive devices become rarer.

Even when wattage is produced on the line, it is not generally full strength. There is still a 'negotiation' process that happens when the device properly identifies which Class it needs.

IMHO, although the number of devices which actually negotiate a power class are increasing, I believe most are still class 0, and draw based on their load once POE is supplied.

sometimes it's simply because there are cheaper passive POE products out there and some people don't know there is a difference between IEEE POE standards and passive POE.

My last comment was to make sure people know that you can get zapped if you're going into an existing system without knowing what's in place and cutting into the wire lets say to re-terminate a cable. In some cases, you can get electrocuted by POE but this shouldn't be possible if it's following the IEEE POE standards.

...In some cases, you can get electrocuted by POE but this shouldn't be possible if it's following the IEEE POE standards.

Unless you just happen to present a 25k ohm load, which would trigger the flow ;)

Either way, IMHO, you are not getting electrocuted, in the strict sense of the word at least.

This is one reason why POE work doesn't normally require an electricians license.

POTS lines can give you a little buzz as well, if you are lucky enough to be a path right when a call comes in.

Agreed, I should have said shocked. I want to edit my response but it won't let me. I will say I've been shocked by POE and it caught me off guard while standing on a tall ladder.

Had one in my mouth once when a call came in. Cured me of that habit!

Because it's cheap. Why bother with all that troublesome negotiation stuff when you can just put 48V across pairs 1 and 4, and let'er rip?

Been there done that - dont plug a 24V UBNT PoE Device into a toughswitch configured for 48V - they kind of stop working

My Smart TV's (former) network port feels your pain.

Passive Aggressive.

The device must be powered to request the power? How does the negotiations work?

In many cases Layer 2 LLDP is used, where the PSE sends a brief 'check' signal of something like 0.05W to the device so it can respond as needing PoE.

This power signal is dissipated by the jack/NIC as harmless if not acknowledged.

Thanks - that would make sense. So in effect it delivers a tiny amount of power by default to boot the Poe controller. It then states its requirements and the switch says ok here's xx watts.

cheers Brian 👍🏻

So in effect it delivers a tiny amount of power by default to boot the Poe controller. It then states its requirements and the switch says ok here's xx watts.

No, not exactly.

The switch delivers a ~2v pulse to test the resistance of the camera.

If the resistance is between 19k and 26k ohm, it either supplies

  1. 48-57v and up to 15w to the camera immediately
  2. or performs additional checks at 20v to determine its power class (if power class >0 is supported by the device), and then supplies power in that range.

In either case, the camera doesn't start booting until this is complete. Also as I mentioned earlier, most Poe af cameras that I have dealt with are just class 0, so they only do step 1.

There is a higher level lldp negotiation that can occur also, but I haven't encountered it in practice.

Login to read this IPVM report.
Why do I need to log in?
IPVM conducts unique testing and research funded by member's payments enabling us to offer the most independent, accurate and in-depth information.

Related Reports

Risk of Amazon Alexa Guard: No Battery Or Cell Backup on Jun 20, 2019
Amazon positions its Alexa Guard Service as a "smart home security system" and says it can help you "keep your home safe". However, the...
China / US Trade War Impact Splits Industry on Jun 04, 2019
As the trade war continues to heat up, 170+ integrators told us "What impact will the US / PRC China conflict have on the industry?" Respondents...
Access Control Job Walk Guide on May 22, 2019
Significant money can be saved and problems avoided with an access control job walk if you know what to look for and what to ask. By inviting...
LifeSafety Power NetLink Vulnerabilities And Problematic Response on May 20, 2019
'Power supplies' are not devices that many think about when considering vulnerabilities but as more and more devices go 'online', the risks for...
Maglock Selection Guide on May 16, 2019
One of the most misunderstood yet valuable pieces of electrified hardware is the maglock. Few locks are stronger, but myths and confusion surround...
Access Control Request to Exit (RTE) Tutorial on May 13, 2019
For access controlled doors, especially those with maglocks, 'Request to Exit', or 'RTE' devices are required to override electrified locks to...
Dahua ePoE Long Distance UTP / Coax Tested on May 03, 2019
Dahua's Enhanced PoE (ePoE) line is claiming extended video and power transmission over 600m without repeaters, with devices interchangeable via...
Register Now - Fall 2019 IP Networking Course on May 02, 2019
Register for the Fall 2019 IP Networking Course. For early registration save $50 off the course's normal $299 price. This is the only networking...
Subnetting for Video Surveillance on Apr 30, 2019
This guide explains when subnetting is used on security networks, and how it works. We explain how to add or remove IP addresses to your range,...
Locking Down Network Connections Guide on Apr 23, 2019
Accidents and inside attacks are risks when network connections are not locked down. Security and video surveillance systems should be protected...

Most Recent Industry Reports

HID Mobile Tested on Jun 21, 2019
HID Global is one of the largest access brands, but their mobile access has had challenges. Indeed, the company has already restructured their...
Genetec Beats Milestone For IHS #1 on Jun 21, 2019
For years, Milestone has touted that they are the #1 VMS. Now, Genetec has beaten them in IHS rankings. But what is this? Even other manufacturers...
Risk of Amazon Alexa Guard: No Battery Or Cell Backup on Jun 20, 2019
Amazon positions its Alexa Guard Service as a "smart home security system" and says it can help you "keep your home safe". However, the...
Exacq Remote Cloud Access Tested on Jun 20, 2019
Remote cloud access has been missing from most VMSes (including Exacq and Milestone). Now, Exacq, after releasing Cloud Drive Storage earlier in...
Briefcam Buys Frost Award* on Jun 20, 2019
Frost 'awards' are well-known and widely disrespected. Now Briefcam is touting their win. The way it has worked for many years is that Frost...
IFSEC 2019 Show Report on Jun 19, 2019
The UK's largest trade show, IFSEC, is underway and IPVM has been examining what is new and happening at the show. Inside, we cover: Huawei...
Repositionable Multi-Imager Camera Shootout - Avigilon, Axis, Dahua, Hanwha, Hikvision, Panasonic, Vivotek on Jun 19, 2019
Repositionable multi-imager cameras are one of the fastest growing segments in video surveillance, with a slew of new offerings being recently...
Genetec Synergis Cloud Link - Complex, Costly and Confusing on Jun 18, 2019
Genetec's Synergis Cloud Link is complex, costly and confusing compared to competitor access control architectures. Inside this note, we examine...
Startup Vaion Launching End-to-End AI Solution Backed with $20 Million Funding on Jun 17, 2019
An EU / USA video surveillance startup, Vaion, founded by ex-Cisco Senior Directors is launching an end-to-end VSaaS platform with $20 million in...
Biometrics Usage Statistics 2019 on Jun 17, 2019
While face and fingerprint recognition are used regularly for smartphones, it is not as common in physical security. In this note, we examine...

The world's leading video surveillance information source, IPVM provides the best reporting, testing and training for 10,000+ members globally. Dedicated to independent and objective information, we uniquely refuse any and all advertisements, sponsorship and consulting from manufacturers.

About | FAQ | Contact